Proportional pressure regulating valves are very frequently used in mobile work machines for electrohydraulic pilot control of directional control valves. Regularly, directly controlled proportional pressure regulating valves are sufficient for smaller work machines. In the case of large work machines or construction machines such as, excavators, the directional control valves of the working hydraulics reach piston diameters, in which a pilot control with directly controlled proportional pressure regulating valves is no longer practical. The large slide valves cause a pilot oil flow during rapid switching, which is no longer, which then exceeds the capacity of the small pilot valves.
For such applications, proportional pressure regulating valves with surface ratios are used. These valves have a pressure-active surface that is smaller than the piston diameter. Thus, the forces caused by the regulating pressure, which act against the magnet system, are significantly reduced. However, the large piston is able to allow a significantly larger volume flow to pass as compared to the directly controlled valve, and therefore, shorten significantly the switching times of the larger slide valves.
This principle of surface ratio has the great advantage, compared to a classical pilot-controlled proportional pressure regulating valve. There is no need for a permanent pilot oil flow. The leakage of such valves, in particular in the non-energized state, is extremely small. This characteristic has significant advantages in the case of an emergency supplying of the proportional valves with a pressure accumulator.
A valve representative of this kind according to the teaching of EP 1 625 307 B1, in particular, is in the form of a proportional seat valve or slide valve having a valve housing and at least three fluid connections extending through the valve housing. A main piston is guided in the valve housing, with a pilot piston effecting a pilot control. The pilot piston may be controlled by an energizable magnetic device. During an open pilot control, fluid from one of the two connectors controllable by the main piston reaches the third connector, controlled by the pilot piston, via a cross-sectional narrowing in the main piston and the pilot control. As a result of the corresponding pressure drop, the main piston in each case reaches a control position, controlling both fluid connections with regard to fluid amount. A compression spring is disposed between the main piston and the pilot piston. The piston stroke of the main piston is proportional to the magnetic flow of the magnet device during open pilot control. The compression spring engages in a recess of the main piston, into which recess the cross-sectional narrowing opens in the form of an aperture.
Because, in the known solution, a contact piece is disposed on the free end of the compression spring assigned to the pilot piston, which is connected to the free end of the pilot piston by a contact ball, an improved interaction between pilot piston and main piston is created. In particular, a largely obstruction-free and, therefore, functionally reliable operation is ensured.
Another valve, in particular, a pressure regulating valve, is known from EP 1 226 478 B1. This valve has a valve housing with at least one pump connection, utility connection and tank connection. A valve piston, which can be controlled by a magnet armature, is guided inside the valve housing. The valve is provided with a hydraulic damping device, which has a damping chamber that is fluidically connected via a choke with a choke point to the utility connection. The pump connection or the tank connection is selectively joined to the utility connection via a connecting line, depending on the position of the valve piston.
Because, in this known solution, the choke point is at least partially formed by an annular gap, which is formed between an annular disc and sections of the valve housing which surround the annular disc, because the annular disc is crimped at points with the valve housing, and because the annular gap is interrupted at the connection points as well as by the front-end contact of the annular disc with the interior of the valve housing, the fluid stored in the damping chamber is displaced via the choke in the direction of the utility connection in the event of a deflection of the valve piston in the direction of the choke caused by the magnetic force of the gap magnet. The displacement volume flow via the choke generates a local pressure build-up.
A force, which is directed against the deflecting force of the valve piston and exerts a damping effect on the entire valve piston, may be determined on the pressure-active surface that defines the choke point. When retracting the valve piston in the opposite direction, this fluid volume coming from the utility connection is then forced to flow back into the now expanding damping chamber again via a defined choke, and results in a damping of the vibrations occurring. An improved control behavior then results for the known valve, which also enhances the functional reliability.
In spite of these advantageous feature configurations in the prior art relating to the valves described above, in particular, in specific applications, the frictional forces of the main piston may no longer be compensated for by the surface ratio.
The application, in which the valves must be maximally energized for very long periods and the primary pressure is less than the maximum possible regulating pressure (undersupply), must be especially critically evaluated. If, in such case, dirt particles end up in the operating clearance of the respective valve piston, the result is a significant increase in friction. If, after this holding period at high energization, the flow is reduced again, a “valve blockage” may then result, because under certain circumstances the relatively small spring force of the return spring is no longer sufficient for returning the piston. This case must be classified as critical to safety, because the main slider controlled by the proportional pressure regulated valve can then no longer be switched back or returned to the neutral position. Thus, the consumer can no longer be stopped.